1. No category

John von Neumann Institute for Computing
Ab Initio Study of Pr Oxides for
CMOS Technology
Jarek Da¸browski, Victor Zavodinsky
published in
NIC Symposium 2004, Proceedings,
Dietrich Wolf, Gernot Munster,
¨
Manfred Kremer (Editors),
John von Neumann Institute for Computing, Julich,
¨
NIC Series, Vol. 20, ISBN 3-00-012372-5, pp. 171-180, 2003.
c 2003 by John von Neumann Institute for Computing
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http://www.fz-juelich.de/nic-series/volume20
Ab Initio Study of Pr Oxides for CMOS Technology
Jarek Da¸browski1 and Victor Zavodinsky1,2
1
IHP, Im Technologiepark 25, 15236 Frankfurt (Oder), Germany
E-mail: [email protected], [email protected]
2
Institute of Materials Sciences, 153 Tikhookeanskaya
680042, Khabarovsk, Russia
We performed ab initio pseudopotential DFT calculations providing insight into the atomic
structures and processes responsible for the quality of alternative gate dielectrics in deep sub100 nm CMOS technologies. Here we summarize our results on point defects in Pr2 O3 , on the
interface structure between Pr oxides and Si(001), and on the interface oxidation and formation
mechanism of the interfacial layer.
1 Introduction
Scaling of Complementary Metal-Oxide-Semiconductor (CMOS) devices requires gate
oxide-equivalent thickness teq < 1.5 nm for the 70 nm technology node. Experts predict that integrated circuits containing such small devices will be on the market already
by the year 2008. Since direct tunneling increases exponentially with decreasing oxide
thickness, gate dielectrics with teq significantly below 2 nm can be of practical use only
if an alternative insulator with dielectric constant K of 20-40 is introduced (the dielectric
constant of SiO2 is ∼4), so that the physical thickness of the insulator can be increased1 .
A key parameter for circuit designers is the current which can flow through the transistor;
this current is proportional to the gate capacitance C, thus defining the required value of
C. The latter can be realized by using a classical SiO2 gate oxide with physical thickness
tox = Kox /C or an alternative dielectric with the dielectric constant K and physical thickness t = K/C. This means that by increasing the dielectric constant n times one can
increase n times the physical thickness of the insulator. Hf and Pr oxides are among the
major candidates for high-K gate dielectrics. Here we focus on Pr2 O3 2–6 .
2 Motivation
The purpose of the calculations is to assist development of gate dielectrics for new generations of MOS transistors1 . The primary practical issue is to achieve the Equivalent Oxide
Thicknessa , EOT as low as 1 nm and less. EOT is associated with general features of
the film morphology. In particular, the chemical composition of the film is a function of
the distance from the substrate8, 7, 9, 10, which affects the effective dielectric constant of the
film. In addition to EOT reduction, the density of charge traps (fixed and reloadable) in
the film and at the interface must be brought down to a value comparable to that typical
for industrial quality gate SiO2 . Charge trap densities are a derivative of film morphology,
a Thickness
of SiO2 with the capacity equal to the capacity of the alternative dielectric layer.
171
being associated with particular processing steps needed to produce a film with a given
stoichiometry distribution, and with the structure and composition of the film.
Reduction of the EOT to the target value is a major technological and scientific problem. EOT is increased by oxidation of the substrate during the growth and by the presence
of a silicate layer at the interface (Pr silicate has a noticeably lower dielectric constant than
Pr2 O3 ). To this end, we have addressed several issues. We have:
• established a low-energy model of chemically sharp Pr2 O3 (110)/Si(001) interface;
• analysed energetics of substrate oxidation in the pre-amorphous regime;
• examined structural models for quasi-crystalline interfacial silicates;
• approximated the range of O chemical potential where Pr2 O3 and SiO2 mix;
• proposed a conceptual model for the interfacial silicate formation.
The second group of problems has to do with the need to reduce the density of charge
traps (reloadable and fixed) to the value comparable to that typical for industrial quality
gate SiO2 /Si(001). The physical origin of these difficulties is less clear at the moment. We
have made an attempt to estimate the viability of various types of defects (including Si
dangling bondsb ) as charge traps. So far, we have:
• computed electrical activity and formation energies of several point defects in Pr2 O3 ,
• considered several interfacial defects involving Si dangling bonds,
• estimated the misfit defect density at the interface given total energy differences and
force constants available from the calculations.
3 Approach
The calculations were done by the ab initio pseudopotential plane wave code fhi96md11.
We applied the Local Density Approximation (LDA) for the exchange and correlation energy12, 13 and nonlocal pseudopotentials in the Trouller-Martins scheme14, 15 with 40 Ry
cutoff for plane waves. The Brillouin zone was sampled at the special k-point set corresponding to the (1/4, 1/4, 0) point from the first Brillouin zone of Si(001) 3×3 surface
cell (interface calculations) or at the Γ point of the cubic Pr2 O3 80-atom cell (bulk calculations). Pr2 O3 /Si(001) films were modelled by periodically repeated slabs consisting of
six Si layers and up to four layers of oxide. The Si substrate was terminated on one side
˚ of vacuum. We computed total enerby hydrogen and the slabs were separated by 10 A
gies and electronic structures for numerous interface and film structures and for various
stoichiometries.
Because of the open f -shell of Pr atoms, a key problem in calculations involving Pr
is in construction of a reliable Pr pseudopotential3, 16, 17. It turns out that two different Pr
pseudopotentials are needed: a pseudopotential with two core f electrons for Pr2 O3 (trivalent Pr(III), +3 ionic charge), and with only one f electron for PrO2 (tetravalent Pr(IV),
+4 ionic charge). Thus, Pr+3 and Pr+4 are treated by us as distinct species. We calibrate
the pseudopotential energy difference such that the experimental difference in formation
enthalpies of Pr2 O3 and PrO2 is reproduced18. The fundamental bulk properties (lattice
constant, bulk modulus) of Pr2 O3 and PrO2 obtained with these pseudopotentials are in
agreement with experimental data; the discrepancies are well within the range typical for
LDA calculations.
b It
is known that Si dangling bonds are responsible for reloadable traps at SiO2 /Si interfaces
172
4 Bulk Pr Oxides
The simplest Pr oxide is PrO2 . It crystalizes in the CaF2 structurure which can be viewed
as the zinc blende structure with anion sites occupied by Ca (or praseodymium) atoms, and
the cation and tetrahedral interstitial sites occuped by F (or oxygen) atoms. Thus, every Pr
atom is eightfold-coordinated and every O atom is fourfold coordinated. All Pr atoms in
PrO2 are Pr(IV) and each has transferred four electrons to oxygen atoms. When a quater
of the oxygen atoms is removed from the crystal, this charge transfer is reduced to three
electrons per metal atom and Pr2 O3 with Pr(III) is formed. The atomc structure of Pr2 O3
remains closely related to that of PrO2 : the oxygen vacancies are ordered in a 2×2×2
simple cubic array made of Pr fcc cubes, each fcc cube containing two O vacancies. In this
way, every Pr atom has now six O neighbors and every O atom retains four Pr neighbors.
The typical composition of praseodymium oxide is Pr6 O11 , corresponding to an ordered (Pr2 O3 )·4(PrO2 ) phase20 . Cubic Pr2 O3 is stable under high vacuum21. When deposited on Si(001) by electron beam evaporation, it grows with the (110) axis normal to
the substrate. PrO2 films on Si have not been reported. Therefore, our calculations are
done nearly exclusively for Pr2 O3 ; the dioxide is treated only as a reference to calibrate
the pseudopotential for tetravalent Pr. This is necessary because one cannot apriori exclude that Pr(IV) atoms appear in certain structures of the interfacial region. However, up
to now we have not identified any structures containing Pr(IV) which are stable under the
conditions during growth and annealing of MBE Pr oxide layers.
˚ less by 0.33% than the experiThe calculated lattice constant of Pr2 O3 is 11.074 A,
˚ 19 , and the bulk modulus is 280 GPa. LDA calculations yield 2.0 eV for
mental 11.115 A
the offset between bulk Si and bulk Pr2 O3 valence band maxima.
5 Formation of the Interfacial Layer
A good matching between lattice spacings of the oxide and of the Si(001) substrate occurs
when the (110) axis of the oxide is normal to the substrate, and the (100) axis of the film is
parallel to the (110) axis of the substrate. In this configuration, each three Si atoms find a
corresponding pair of Pr atoms. Two of these three Si atoms become now dimerized, which
leaves the Si surface with four dangling bonds per each 3×1 unit, that is, two Si dangling
bonds per each Pr atom on the oxide side. If the oxide had the composition of PrO2 , this
would mean that there are four oxygen atoms per these two Pr atoms; the position of these
O atoms would roughly match the position of the dangling bonds bonds and each of the
interfacial Si atoms could be oxidized. However, in Pr2 O3 stoichiometry there are only
three O atoms available for this purpose, leaving in each 3×1 unit one Si dangling bond
without an oxygen partner.
An additional important factor is a charge mismatch between these materials. Bulk
Pr2 O3 is strongly ionic: one can assume that each Pr atom gives three electrons away
and each O atom captures two electrons. But since O atoms at the interface form bonds
(predominantly covalent) with Si atoms, they are only partially involved in the charge
transfer from metal atoms. As a consequence, each interfacial Pr2 O3 moiety donates two
electrons which cannot be localized on the existing anions if the oxide is stoichiometric.
We find that in the ideal case of pure Pr2 O3 /Si(001) film (no silicate) these electrons
are trapped by additional O located in the second Pr layer. This basic model, constituting
173
a)
b)
Figure 1. Fundamental interface structures of Pr2 O3 /Si(001). Si atoms are red, O atoms are yellow, Pr atoms are
blue. Electron densities of state are semiconducting, with clean gap. (a) Stoichiometric interface. This system
has one Si dangling bond (electrically inactive) per interfacial Pr atom, or two per 3×1 unit. The interfacial Pr
layer has 1/3 deficiency in oxygen, the next Pr layer is enriched in oxygen by 1/3, and the remaining layers are
stoichiometric. In Fig. 2, this structure is labelled 2(SiPr). (b) Interface enriched with oxygen. The dangling
bonds are oxidized, and both the interfacial and the next-to-interfacial Pr layers are rich in ogygen; nevertheless,
all Pr atoms are in Pr+3 charge state. In Fig. 2, this structure is labelled 0(SiPr).
the starting point of analysis of the system, is illustrated in Fig. 1a for stoichiometric oxide
and in Fig. 1b for oxygen rich oxide. In stoichiometric Pr2 O3 /Si(001) one of the interface
O atoms moves from Si–O–Pr site into the film, leaving behind a SiPr “bond”, i.e., a Si−1
ion forming a mostly ionic bond to two Pr+3 neighbors (Fig. 1a). This dangling bond
is not a source of trapped charge, because the electrostatic interaction with the positively
charged neighbors moves the (0/-) transition state into the valence band of silicon.
5.1 Energetics of Substrate Oxidation
During MBE deposition by electron beam evaporation from a Pr oxide source, the growing
film is in contact with vapor consisting of PrO molecules and O atoms. In practice, this
means that the system is exposed to an oxidizing envoronnemt. Indeed, noticeable Si
oxidation takes place unless the substrate temperature is so low that the Pr oxide film grows
amorphous. When the tepmerature is high enough for epitaxial growth (e.g., 500o C), some
oxygen atoms arrive at the substrate and react there.
In this Section we discuss oxygen incorporation energies at various interfacial sites.
We also consider oxygen-poor stoichiometries.
Fig. 2 collects formation energies of many interfacial structures, shown as a function of
oxygen chemical potential µ(O). Zero of the formation energy refers to the fully oxidized
boundary (Fig. 1b), and zero of µ(O) is chosen at equilibrium with SiO2 . The number of
Pr atoms is the same in all structures, but in general, thermal equilibrium requires that:
µ(Pr) + 1.5µ(O) = ∆Gf (PrO1.5 ) − 0.5∆Gf (SiO2 ),
(1)
where the ∆Gf stands for Gibbs formation energy of a compound and the term
0.5∆Gf (SiO2 ) is added because the standard formation energy of Pr2 O3 is measured with
174
Figure 2. Stability of Pr2 O3 /Si(001) interface structures as a function of oxygen chemical potential µ(O). Note
that zero of the chemical potential is chosen at the equilibrium with SiO2 ; 0.5µ(O2 ) is thus far on the right-hand
side, at µ=4.9 eV (LDA value). Bold lines correspond to the structures discussed in the text. The labels associate
each structure with its characteristic feature: n(SiPr) means that there are n SiPr interfacial units per each 3×1
unit, and n(SiOSi) means that there are n units on the Si side.
respect to O2 , not SiO2 . The left-hand end of the diagram represents the oxygen-poor limit,
roughly corresponding to µ(Pr)=0, that is, to the equilibrium with Pr metal.
Let us go through Fig. 2 from the oxygen poor to the oxygen rich limit, focusing on
the structures with the lowest energies. The interface labelled 4(SiPr) has four SiPr units
in each 3×1 cell, that is, there is no oxygen between interfacial Si and Pr atoms. These
interfacial SiPr sites are the first to be oxidized. The energy of oxygen incorporated into
such a site depends on the local geometry (dimerized or undimerized Si atom) and on
the total number of incorporated oxygen atoms and varies between -1.2 and -0.5 eV. Si–Si
bonds are oxidized next. Due to geometry constraints, O incorporated into a surface Si–Si
bond has energy by ∼0.6 eV higher than in SiO2 . This can be viewed as the onset of SiOx
formation. Oxidation of subsurface Si–Si bonds leads to further stress accumulation, Si
ejection, and eventually to formation of amorphized SiO2 interfacial layer.
5.2 Silicide Formation
Although we did not calculate silicide structures explicitely, the left-hand side of the diagram in Fig. 2 strongly suggests the stability of the film with respect to silicide formation.
This conclusion comes from a simple comparison of formation energies computed for interfacial structures containing SiPr bonds (silicide type) and PrOSi bonds (silicate type).
A stoichiometric interface has 50% of the interfacial bonds of silicide and 50% of silicate
type (Fig. 1a). O atoms are removed from the film in the oxide poor regime, when µ drops
below µPrOPr = -2.3 eV. At more oxidizing conditions, when µ rises above µPrOSi = -1.0
eV, oxygen is inserted into interfacial PrSi bonds. Since the energy of O is lower between
175
Figure 3. Stability of silicate structures at the Pr2 O3 /Si(001) interface as a function of oxygen chemical potential
µ(O). Bold lines correspond to the structures discussed in the text. The labels associate each structure with its
characteristic feature: buck:SiO2 and flat:SiO2 refer to SiO2 molecules dissolved in an ultrathin film unrer a
buckled and a flat surface (Fig. 4a-b), “PrOSi” indicates the presence of mostly PrOSi bonds in an intercalating
SiO2 layer (Fig. 4c) and “PrOSiSi” indicates that SiSi bonds occur in a thicker intercalating SiO2 layer (Fig. 4d).
The structures 0(SiPr) and 4(SiOSi) are the same as in Fig. 2.
Pr and Si atoms than between two Si atoms (by 1.0 eV), silicide can be formed only when
not enough O is available. Otherwise, O is extracted from SiOSi bonds and inserted between Si and Pr, reducing the Si (sub)oxide and oxidizing the silicide.
5.3 Silicate Formation
We now consider the possibility of silicate formation at the interface. We computed total
energies for numerous simplified models of interfacial silicates, differing in stoichiometry, atomic arrangement, and imposed lateral periodicity (from 3×1 to 3×3, measured in
Si(001) surface lattice constant). Most of these structures are clearly unstable, that is, their
energies fall well above the lowest energies of silicate-free film at any realistic value of
oxygen chemical potential. However, some of the structures turned out to be stable for O
chemical potential in the SiOx range (Fig. 3). This is in spite of the fact that the small cell
sizes used (for practical reasons) in the calculation have most certainly lead to accumulation of lateral stress in the silicate layer.
When a SiO2 molecule dissolves in Pr2 O3 , two O−2 atoms are substituted by a
(SiO2 )−4 moiety. In other words, two O atoms are removed from the film to the reservoir of O with energy µ, a Si atom is added from Si bulk, and four O atoms are taken from
the reservoir and placed between Pr and Si. If Si is in equilibrium with Si bulk and O is in
equilibrium with the reservoir, we can thus estimate the silicate formation energy Ef from
Ef = (2µPrOPr − 2µ) + (4µ − 4µPrOSi ) ' 2µ − 0.6eV,
176
(2)
a)
b)
c)
d)
Figure 4. (a) A SiO2 molecule buckles the surface of the ultrathin Pr2 O3 film. This structure is labelled
buck:SiO2 in Fig. 3. (b) Filling the trenches with 0.5 monolayer Pr2 O3 flattens the surface and reduces the
energy. This structure is labelled flat:SiO2 in Fig. 3. (c) A monolayer of Si oxide intercalated above the first
Pr2 O3 layer. Most of the bonds of the intercalate are of silicate character. This structure is labelled PrOSi in Fig.
3. (d) . Additional two monolayers of Si suboxide intercalated above the first Pr2 O3 layer. There are SiSi bonds
in the intercalate. This structure is labelled PrOSiSi in Fig. 3.
where the energy of O in the film is approximated by µPrOPr . Positive Ef means that the
silicate is stable. It follows that already at µ ' 0.3 eV a process collecting two O atoms
on a Si atom and dissolving the resulting SiO2 moiety in the Pr2 O3 film is expected to be
energetically favorable, due to low energy of oxygen between Pr and Si atoms (µPrOSi ).
The estimate of Eq. 2 is confirmed by a direct calculation for a SiO2 molecule dissolved in a 3 monolayer thick Pr2 O3 /Si(001) film. Silicate formation begins at µ1 = 0.8 eV
when the film surface buckles due to the additional volume introduced by the SiO2
molecule, and at µ2 = 0.3 eV when the surface is made smoother by depositing half a
monolayer of Pr2 O3 (Fig. 4a-b). This formation energy difference turns out to be equal
to ∆E0.5 = E4 − E3.5 = E3.5 − E3 , where En is the energy of the film with n atomic
layers of Pr2 O3 .
High concentration of Si in the silicate is stabilized if the Si atoms are intercalated
between Pr2 O3 planes. An example of such a structure, with Si/Pr ratio of 3, is shown
in Fig. 4d. The structure is stable above µ ' 0.5. Note that not all SiSi bonds in the
silicate are oxidized. Oxidation of all these bonds may be difficult without simultaneous
excessive oxidation of the substrate: the energy gain is comparable for both processes and
the concentration of SiSi bonds is much higher in the silicon than in the silicate.
177
The presence of such a partially ordered silicate explains the recovery of the substratedetermined orientation in the Pr2 O3 film grown on top of the apparently amorphous interfacial layer. This recovery is clearly visible in TEM images and in XRD rocking curves.
How does SiO2 enter the growing film? We found that SiO2 moieties are stable on
the surface of a single Pr2 O3 monolayer if the oxygen potential is in the range of the
oxygen energies in the surface Si–Si bonds. They can be overgrown with Pr2 O3 , leading
to silicate formation from the very begining. Comparison of energies and geometries of
other structures computed by us indicates that the overgrowth process may be complicated:
when the growth of a Pr2 O3 plane is not yet complete and the surface is rough, or when
the amount of SiO2 is not high enough to build an intercalating plane without inducing a
strong deformation to the capping Pr2 O3 plane, then SiO2 moieties tend to segregate to the
surface. After the topmost Pr2 O3 plane is closed or enough SiO2 is collected, the silica
units move under the surface in order to maximize the number of Pr–O–Si bonds.
6 Defects and Charge Traps
Reloadable charge can be trapped by electrically neutral defects which have electron transition levels in the forbidden energy range. A Si dangling bond, if not associated with Pr,
acts as this kind of trap. Also oxygen vacancy in Pr2 O3 introduces a reloadable trap state.
Life time of charge localized on such trap sites at the interface or in an ultrathin film is limited because the carrier can tunnel away to the substrate. Fixed charge can exist even in an
ultrathin film when the relevant transition levels are degenerate with Si bands. Examples
of such fixed charge traps in Pr2 O3 are Pr vacancy, Pr interstitial, and O interstitial.
We calculated the formation energies and electronic structures of fundamental point
defects in Pr2 O3 . The Frenkel pair formation energy is about 5 eV in the metal sublattice
and 6 eV in the oxygen sublattice. Pr vacancy introduces states close to the valence band
(VB) top and may exist in -3 charge state. Pr interstitial introduces states close to the
conduction band (CB) bottom and may exist in +3 charge state. O interstitial introduces
states close to the VB top and may exist in -2 charge state. Finally, O vacancy introduces
midgap states and may exist in +2 charge state.
We will now roughly estimate the lower limit of the interfacial density of misfit Si dangling bonds. We approximate the film/interface/substrate system by a sum of three components: (1) completely relaxed film with rigid lattice, (2) partially relaxed interfacial layer
of a given thickness, with lattice constant matching a certain multiplicity of the substrate
lattice constant, and (3) rigid substrate. The strained layer is assumed to be compressed,
so that one expects formation of dangling bonds in the substrate (the substrate has more
surface orbitals than needed for bonding with the layer). Moreover, we assume that all
atomic planes in the layer relax in identical way. We also ignore misfit defects between the
strained layer and the relaxed film, because the need to create such defects only increases
the effective stiffness of the layer, facilitating the formation of Si dangling bonds. Elastic
energy of this system can be written in terms of elastic constants of the film (energy stored
in the layer), force constant for deviations from the ideal alignment of “connecting points”
between the layer and the substrate (interface “friction” energy), and the matching period
of the substrate and the strained layer. The energy change upon relaxation enabled by an
array of mismatch defects should be compared to the formation energy of such a defect.
178
Node displacement, in fraction of lattice constant
0.5
relaxation energy: −3.77 eV
strain energy density: 0.68 eV/site
DB density: 1e+013/cm2
substrate surface
0.4
2*Nd: 60 sites, 3 ML film
Ks/Kf: 0.017 = 1.41/(3*28.31), k in eV/A2
average strain: −1.71%
strain range: (−3.07%, 2.88%)
0.3
0.2
0.1
0
film bulk
−0.1
0
5
10
15
20
Lattice site away from the defect
25
30
Figure 5. Interface friction model of dangling bond creation at Pr2 O3 /Si(001) interface. Crosses indicate the
strain at each connection node. Blue crosses refer to strain in the film, red crosses indicate the strain in the
interfacial plane. The interface friction constant has been obtained for the chemically sharp interface 0(SiPr),
Fig. 1b. Note that the strain field induced by the defect (node 0) extends only to 20–30 nodes.
˚ an order of magnitude
The computed friction force constant is small (ks =1.4 eV/A),
smaller than typical bond stretching constants in covalent crystals. Assuming that the interfacial Pr2 O3 is described by the bulk elastic constants, that the lateral relaxation proceeds
along one direction, and approximating the plane-strain force constant of the film by
kf =
BAlatt
˚ 2,
= 28.31 eV/A
3(1 − ν)(1 − 2ν)
(3)
˚ are computed values and ν is the Poisson ratio
where B = 280 Gpa and Alatt = 11.07 A
assumed to be 1/3, we conclude that while a 2 monolayer (ML) thick film cannot relax
through interfacial defect formation, a 3 ML film stores enough energy to create one misfit
defect (with formation energy of 1–2 eV) per about 15 lattice sites (2·1013 cm−2 . Fig. 5
illustrates that relaxation around an interface dangling bond extends to about 20–30 lattice
sites; this is the reason why a 2 ML film cannot relax through interfacial defects.
This result means that a Pr2 O3 film grown directly on Si(001) would induce an unacceptably high density of interfacial defects. Therefore, an interfacial silicate layer seems
to be necessary. The stress may be relieved in the intercalate planes without formation
of interfacial dangling bonds. However, the Si content in the silicate should be kept low
enough (Pr/Si ratio ∼ 1) to eliminate the hazard of formation of Si–Si bridges in the film.
7 Summary and Conclusions
We performed ab initio total energy calculations for Pr2 O3 bulk and Pr2 O3 /Si(001) interfaces, obtaining a number of data helpful in the understaining of the processes responsible
for the electrical quality of the films produced for CMOS applications, and providing some
guidance for further experimental work.
179
We obtained fundamental information on native point defects in bulk Pr2 O3 . We found
that Pr vacancies and O interstitials would introduce fixed negative charge in the film, Pr
interstitials would introduce fixed positive charge, while O vacancies would be amphoteric.
We established a fundamental atomic model for the interface and we examined the
energetics of the interface oxidation and reduction. We found that SiO2 moieties are thermodynamically unstable in SiOx under Pr2 O3 (110)/Si(001) and dissolve in the Pr oxide. It
is plausible that the structure of the resulting silicide is quasi-crystalline and intercalated,
with Si/Pr ratio as high as 3. Calculation of interfacial friction between Pr2 O3 and Si(001)
allowed us to estimate the lower bound for interfacial misfit dangling bonds when the film
contains no SiO2 admixture. We conclude that such films would have an unacceptably high
density of interfacial dangling bonds. A silicate layer is expected to improve the situation,
but the Si content in the silicate should be kept around 1 to avoid the hazard of Si–Si bridge
formation in the film, since such bridges can be broken and trap charge.
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